A detailed understanding of interplay between genetics and epigenetics in the progression and hormone dependence of distinct breast cancer subtypes is critical to the development of new approaches to prevention and therapy. To address this problem we have begun to explore the epigenetic state of the normal mammary epithelial cells and of tumor cells. We have used cell surface markers and fluorescence? activated cell sorting of normal mammary cells from ostensibly normal women undergoing reduction mammoplasty to operationally define mature luminal, luminal progenitor and basal/mammary stem cell? enriched populations. In Aim 1 we will test the hypothesis that estrogen receptor (ER) signaling in breast cancer recapitulates ER signaling found in luminal progenitors. An understanding of the ER-regulated program in normal luminal progenitors has important implications for both hormonal chemoprevention and for endocrine therapy. We have also begun to investigate the potential impact of BRCA1 and BRCA2 genotype on ER signaling and the epigenetic state of the normal mammary gland. Aim 2 will test the hypothesis that BRCA1 and BRCA2 in addition to their well-documented roles in maintaining genomic integrity play important roles in determining mammary cell fate and that ER signaling influences these effects. In preliminary studies we have found that the genes aberrantly expressed by luminal progenitor cells derived from BRCA2 carriers shows a striking overlap with the gene expression program that we recently defined as playing an important role in the oncogenic function of the epigenetic regulator EZH2 in castration-resistant prostate cancer. Aim 3 will address the hypothesis that EZH2 plays a necessary role' in normal mammary development and an oncogenic role in breast cancer. These three specific aims depend critically on the Pathology Core and the strength of the collaborations supported by this Program Project.